Traditionally, cast iron parts are formed by first casting molten iron into a desired shape, and then machining the iron casting to the desired dimensions. The intended uses of cast iron parts may require that the parts be hardened or tempered in order to prolong the lifetimes of the part, for example, for improving the wear resistance of the parts. In the past, it has often by the practice to temper an entire cast part by through tempering, that is, by fully tempering the part throughout the whole of its body. Through tempering a part entails heating the entire part to an austenitizing temperature, and then quenching the part (for example, by immersion in an oil or molten salt bath) to cause compressive stresses in the material, and thereby harden the part.
Unfortunately, through tempering has been very expensive, due to long cycle times, high energy consumption, and the formation of low quality parts (this last caused by distortion during the heating stages). Moreover, through tempering has been disadvantageous because the shape of a part may become distorted during heating, quenching, or both, and the hardening achieved upon subsequent quenching makes the part very difficult to machine to the desired final dimensions. Accordingly, a number of prior attempts have been made to harden merely the surface of cast iron parts, while allowing their cores to remain untransformed. While such attempts did yield parts in which only the surfaces of the parts were hardened, such methods have been subject to their own drawbacks.
For example, flame hardening and induction heating have been used to locally heat an area on the surface of a part before the part is quenched to achieve hardening of the surface. A variety of heating methods are disclosed as background in U.S. Pat. No. 5,064,478 (Kovacs et al., Nov. 12, 1991), at column 2, line 20 through column 3, line 2. That description is incorporated by reference herein. Such methods are limited in that they are generally not useful for cast parts which have many protrusions or indentations, because flame hardening and induction heating methods cannot uniformly and perpendicularly heat all of the part surfaces at the same time. Manufacturers using these or comparable methods have experienced problems due to uneven heating, non-homogeneous brittleness and low yield in production. Moreover, surface hardened parts produced by these or other comparable methods are inherently expensive and difficult to manufacture or machine.
One potential solution to the specific problem of uneven heating of the surface of a ferrous cast part is proposed in U.S. Pat. No. 4,637,844 (Pfaffmann, Jan. 20, 1987). The disclosed process entails preheating the part to around its desired isothermal transformation temperature and inductively heating the part to obtain an austentizing temperature to a substantial depth within the part, in a short period of time which is urged nonetheless to be effective to promote the desired metallurgical carbon and/or graphite dissolution in the surface layer. The induction heating is asserted to be confined to the outer surface of the part, so as not to significantly raise the temperature of the part interior. After induction heating, the part is immersed into an oil bath maintained at the desired isothermal transformation temperature.
The process disclosed in the patent possesses several drawbacks, however. Induction heating processes are well-known to be non-uniform, that is, they heat the exterior of the surface layer to a temperature significantly greater than that to which the remainder of the surface layer is heated. Moreover, for practical reasons, induction heating of the surface of cast parts is conventionally carried out under an ambient atmosphere, so that oxidation of the surface of the part occurs to a significant degree. Additionally, the nature of induction heating permits only a very short time for austenitizing of the surface layer (at least, short in contrast to other methods), so that the stability of the ausferritic layer ultimately formed has a lower stability than would be desired. Lastly, as with other methods, quenching is carried out with a liquid bath of high temperature oil or molten salt. As indicated above, the need for a quenching bath increases the overall cost of producing the parts.
U.S. Pat. No. 5,064,478 mentioned above provides a solution to many but not all of these problems. That patent discloses a method for producing a selectively surface hardened cast iron part which includes the uniform heating of the surface of the part by immersing the part into a molten metallic bath until only a desired thickness of a surface austenite is produced on the part, the bulk of the body of the part remaining unheated. The surface-heated cast iron part is thereafter quenched in a liquid quenching bath maintained at the desired austempering temperature. Because the body of the part remains well below the austenitizing temperature, the part does not deform during surface hardening, and can be pre-finish-machined to its desired final dimensions before the surface hardening is accomplished. Moreover, unlike prior methods, the method of that patent produced a hardened layer to a uniform depth without regard to the shape of the part, avoiding the need for heating to a substantial depth, as required by U.S. Pat. No. 4,637,844.
While very useful for its intended purposes, the method of U.S. Pat. No. 5,064,478 still incurs the cost and inconvenience of the molten salt bath for quenching. It would be desirable and advantageous to devise a method for hardening only the surface of a cast iron part, one which avoided the use of a liquid quenching bath yet enjoyed the limited and uniform heating of the surface provided in U.S. Pat. No. 5,064,478.
Accordingly, it is an object of the present invention to provide a method for producing a selectively surface hardened cast iron part in which the surface tempered layer is uniformly produced without regard to the shape of the part.
It is also an object of the present invention to provide a method for producing a selectively surface hardened cast iron part in which a relatively longer time for austentizing is employed, in contrast to methods entailing induction heating or flame hardening, so as to improve the stability of the ausferritic layer ultimately formed.
It is a further object of the present invention to provide a method for producing a selectively surface hardened cast iron part in which the part formed has a surface free of oxidation and which does not require post treating, such as machining to finish dimensions.
It is yet another object of the present invention to provide a method for producing a selectively surface hardened cast iron part which does not entail quenching in a molten salt bath or oil bath, thus reducing the cost of manufacturing the cast iron part.